Lithium, the 3rd element of the Periodic Table, the lightest metal and the 32nd most abundant in the earth's crust, is expected to play an important role in the rapid development of batteries for electric vehicles.
In the past few years a sustained increase in the use of lithium beginning with its use in pharmaceutical products in the early 20th century, to the present where it is used in manufacture of ceramics, glasses, aluminum products, synthetic rubber production, chemical products and alloys, as well as in the production of electric batteries. This latter application is expected to result in higher demands than all other uses by the mid 21st century.
Lithium can be obtained from various sources. One of them are brines as found in salt flats, salt water lakes, geysers and salt mines. The chemical composition of these brines varies greatly depending on the source. With respect to lithium, large differences in content as well as associated salts can be found. Table 1 shows some of the chemical compositions of brines from different parts of the world.
TABLE 1Chemical composition of naturally occurring brines (%).OrigenLiMgCaNaKClBSilver Peak, USA0.020.020.716.30.810.10.005Dead Sea, Israel0.0024.00.063.10.616.10.003Ocean (average)0.00010.120.041.050.041.920.0004Salar de Atacama,0.15-0.201-1.40.04-1.55.7-7.21.7-1.916-170.04-0.05ChileSalar de Cauchari,0.050.130.039.80.4915.50.47Argentina
Spodumene, (LiAl(SiO3)2), is an important mineral source of lithium and contains 3.73% lithium. Spodumene is a pyroxene (double aluminum and lithium silicate) and has been an important source material in the production of different lithium compounds and is the principle lithium mineral exported at present. Other minerals exploited commercially are: petalite (LiAlSi4O10) containing 2.27% lithium and lepidolite which has a variable composition. The latter two minerals are used as additives in glass and ceramic production but are not currently used as sources of lithium compounds or metallic lithium.
There are many other minerals that contain lithium, given that lithium is extremely reactive (having a lone electron in its outer layer) and can therefore form compounds with almost all the elements of the Periodic Table. Chlorides, bromides and fluorides of lithium are very soluble in water. Thus explaining that the lithium content of ocean water (10-4%) making it the potentially largest source of lithium in the world.
Treatment of brines obtained from salt flats and salt lakes vary considerably in accordance with their chemical composition. Generally, chloride brines contain significant quantities of magnesium that has to be removed before the lithium is precipitated. Depending on the end use of the lithium or lithium compound, other contaminants which must be removed are boron, calcium and sodium.
Battery grade lithium requires a sodium contamination level below 6×10 −4% due to the fact that that this metal can oxidize violently in the presence of oxygen, thus providing a risk of ignition. Magnesium must also be below 5×10−3% due to the fact that this metal accumulates in the in the electrolyte during the process of electro-winning of lithium via electrolysis of melted salts, thereby short circuiting the cells. Metallic lithium is obtained by using a melted electrolyte containing 55% KCl and 45% LiCl at 800 to 850° C. while under an argon atmosphere.
As pointed out hereinabove, each particular lithium brine can require specific methods of purification. This has led to various processes for such purification. The majority of the patented processes for the chloride brines follow a protocol involving removal of boron via solvent extraction; dilution of the brine with mother liquor; a two stage magnesium precipitation; and final lithium precipitation in carbonate form.
For chloride brines such as one the ones found at the Salar de Atacama, in the north of Chile, U.S. Pat. No. 5,993,759 teaches a process for treatment of pre-concentrated lithium brines that have 5 to 7% lithium, 0.5% boron and 1 to 2.5% magnesium, these latter two elements being the primary contaminants. The described processes involve an initial step of boron removal via the use of solvent extraction. Solvents used in this step are solutions of various aliphatic alcohols in an aromatic solvent solution. The boron depleted brine solution are then diluted with mother liquor yielding a lithium brine containing 0.8-0.9% lithium. This dilution serves the purpose of avoiding excessive lithium precipitation given that the next step is magnesium carbonate (MgCO3) using soda ash (Na2CO3). After the solid-liquid separation, a second magnesium precipitation using slaked lime (Ca(OH)2) resulting in a magnesium hydroxide precipitate. The purified brine is then treated with soda ash at 80-90° C. in order to precipitate the lithium carbonate, a compound that posses a solubility inverse to temperature. The described process concludes with a filtration step followed by heated washing and subsequent drying.
This process, with some changes, has also been suggested for other brines. For example. U.S. Pat. Nos. 5,219,550 and 6,921.522 describe processes similar to the aforementioned one with additional steps that reduce levels of certain impurities, such as calcium and sodium.
Battery grade metallic lithium requires a high purity lithium chloride that can be produced from lithium carbonate or lithium hydroxide. Electrolyte grade lithium chloride requires low level of sodium (0.006%) and low magnesium (0.005%), yielding a lithium carbonate with 99.4% or greater purity.
Lithium carbonate obtained employing conventional methods as described in U.S. Pat. Nos. 5,993,759, 5,219,550, 4,261,960, 4,036,718 and 4,243,392 normally contain 99.2% Li2CO3 with 0.2 to 0.3% sodium and 0.05 to 0.1% magnesium, contamination levels that do not allow for use in the production of battery grade lithium.
There are several patented methods for the production of lithium chloride and lithium carbonate. For example, U.S. Pat. No. 4,980,136 describes a method for producing battery grade lithium chloride from lithium rich brines using solvent extractions with aliphatic alcohol, that is subsequently evaporated leaving high purity crystalline lithium chlorides. Other methods, as described in U.S. Pat. No. 4,859,343 teach the use of ion exchange columns that remove sodium ions from chloride brines.
Given that it is very difficult to produce lithium chlorides with less than 016% sodium directly from lithium carbonates, such carbonates are generally transformed into lithium hydroxide and then into lithium chloride or the lithium carbonate is treated with hydrochloric acid yielding lithium chloride that can be further purified via successive crystallizations. The described methods, even though requiring multiple steps, result in a lithium chloride product which is suitable for electrolysis, having a sodium level below 0.06%.